With respect to motion restoring intervertebral devices see German Patent No. 2,263,842 and the devices disclosed therein. These include ball and socket prostheses with single and/or dual articulations and with and without motion dampening elements. The clinical use of the device containing a ball supported by two opposing sockets extending from generally flat plates, was discussed in a Hoffmann-Daimler's article appearing in Intervertebral Disk Displacement, Vol. 112, No. 4, Aug. 1974. Hansen et al., U.S. Pat. No. 5,676,701 (“'701 patent”) discloses a low wear artificial spinal disc having opposing convex and concave contoured surfaces with a full 360 degree circumference. This allows for rotation but no translation. Yuichiro and Koichi, U.S. Pat. No. 5,899,941 discloses a similar device, but where the articulating surface is generally non-conforming to allow rotation and translational movement. This is similar to the device illustrated in U.S. Pat. No. 6,113,637 by Gill et al. This artificial spinal disc incorporates a ball and trough type articulation with a substantially flat portion. Both the Yuichiro and Gill et al. patents describe devices which rely on point contact, a potential source of excessive wear debris.
Also, see the prior art discussed in the '701 patent and U.S. Pat. No. 5,071,437 (“'437 patent”). These prior art intervertebral devices generally rely on an elastomeric core to provide the articulation, which core is sandwiched between rigid plates abutting the respective vertebral bodies (“'437 patent”) or hard metal ball and socket components interposed between adjacent vertebrae.
The use of a hard/stiff material, generally ceramic or a cobalt chromium alloy, to provide the articulating surfaces decreases the wear and increases the life expectancy of the artificial disc. However, the use of the same material or rigid plates, as in the '437 patent, abutting the faces of the separated vertebrae, results in stress shielding, i.e., a nonuniform distribution of the forces transmitted from the articulating surfaces to the abutted vertebrae bone faces. This nonuniform loading can result in so called hard spots on the bone face where the load is being transmitted and soft spots where little or no load is being transmitted, with the consequence that the soft spots tend to result in bone absorption, thereby loosening the implant and causing eventual failure.
There is a need to provide a softer/less stiff material to interface with the vertebrae bone faces and a hard material to serve as the articulating surfaces. Since a softer material, capable of supporting the articulating layer, may not be conducive for bone attachment, there is a need to modify the vertebrae engaging or buttressing surface of the softer material to facilitate bone attachment. There is a further need to inhibit any slight motion, e.g., macromotion, between the material forming the articulation surface and the material forming the vertebrae engaging or bone buttressing surface to reduce or eliminate any wear at the interface of these materials. Such wear, given time, will cause separation at the interface and shorten the useful life of the prosthesis.
The above problems are also associated with artificial limb replacement joints, except that some joint replacements such as hip joints may, as a practical matter, have a need to relieve the stress imparted only to one of the bone ends, e.g., the bone portion forming the acetabulum. Artificial hip joints typically include a metal cup which houses the acetabular bearing portion, which metal cup is secured, via cement, for example, directly to the bone. See, for example, U.S. Pat. Nos. 6,368,354 (“'354 patent”) and 6,966,932 (“932 patent”). As discussed above, the metal to bone contact provides a nonuniform stress loading to the exposed bone face with a significant potential of causing the implant to loosen from the underlying implant. It is noted that both of the above patents illustrate an interlocking arrangement between several sections of the acetabular component. However, the interface between the outer metal shell and the bone end or face forming the acetabulum does not resolve the nonuniform loading on the bone face. As with the artificial intervertebral joint there is a need to provide stress shielding in artificial hip replacements as well as in other artificial appendage joints.
With respect to item (b) above, a spacer or disc replacement member, made of a material such as Ti, has been used to maintain the vertebrae encompassing a failed natural disc in a separated condition during the fusion process. Currently, spacers formed of a softer material, such as PEEK, have been used for this purpose to provide better stress transfer to the bone during the fusion process. However, PEEK is not particularly bone attachment friendly. There is a need to provide an artificial disc replacement member which accommodates both even stress transfer and bone-on growth attachment.